What are peptides? A plain-English guide

A peptide is a short chain of amino acids, the same building blocks that make up proteins. The difference is length: FDA regulations define a protein as an amino acid chain longer than 40 amino acids, counting all chains that naturally associate, which leaves anything shorter in peptide territory. [1]

That definition sounds technical, but it explains a lot about the peptide market. Some of the oldest and most important medicines in the world are peptides. So are many of the unapproved injectables sold online as "research chemicals." Knowing which is which is the whole job of our peptide index.

Your body runs on peptides

Many hormones and signaling molecules are peptides. A few examples that are also approved drugs:

• Oxytocin is 9 amino acids long. As the injectable Pitocin, it is FDA approved to induce or strengthen labor and to control bleeding after delivery. [3]
• Glucagon is 29 amino acids long. Injected, it is the emergency treatment for severe low blood sugar. [4]
• Insulin has 51 amino acids split across two chains. Because the rule counts both chains together, insulin actually crosses the protein threshold. [1] It has been a life-sustaining therapy for decades, and the landmark DCCT trial showed intensive insulin therapy sharply reduced eye, kidney, and nerve complications in type 1 diabetes. [2]

These examples show the first rule of peptides: "natural" does not mean "harmless." Insulin is essential, yet in the DCCT the chief adverse effect of intensive therapy was a two to three fold increase in severe hypoglycemia. [2]

Engineered peptides: the GLP-1 story

Drug makers modify natural peptides to make them last longer or work better. The GLP-1 class is the clearest case. Exenatide, a 39 amino acid peptide first found in Gila monster saliva, became the first GLP-1 receptor agonist approved in the US in 2005. [6] Semaglutide is a modified version of human GLP-1 with a fatty acid attached so it lasts a week. In the STEP 1 trial of 1,961 adults with obesity, weekly semaglutide 2.4 mg produced an average weight change of minus 14.9% versus minus 2.4% with placebo over 68 weeks. [5]

Engineering also explains why most peptides are injected. Digestive enzymes break peptide bonds, so an unmodified peptide swallowed as a pill is mostly destroyed before it reaches the blood. Oral semaglutide exists because it was specifically formulated to survive that trip.

What are peptides used for?

Approved peptide drugs treat diabetes, obesity, severe low blood sugar, and labor and delivery, as above. Outside approved medicine, peptides are marketed for four main goals, and the evidence behind each is very uneven:

• Weight loss. The GLP-1 drugs have large human trials; most other "fat loss" peptides do not. Our peptides for weight loss ranking sorts them by human evidence.
• Muscle growth and recovery. Growth hormone secretagogues and healing peptides such as BPC-157 rest mostly on animal data. [11] See the peptides for muscle growth ranking.
• Skin and hair. Copper peptides such as GHK-Cu are the best known; see peptides for skin aging.
• Longevity and cognition. Almost entirely animal or small uncontrolled human studies.

Clinics often sell these under the label "peptide therapy." What peptide therapy is explains what that term covers, and how to get peptides prescribed covers the lawful routes.

Three kinds of peptide products

Almost every peptide you will read about falls into one of three regulatory groups, and the group matters more than the marketing.

1. FDA approved drugs

These went through clinical trials and FDA review, and are made in inspected facilities with a known label. Semaglutide, tirzepatide, oxytocin, glucagon, and insulin are all here. Our pages show the approval dates and label indications for each.

2. Compounded peptides

A licensed pharmacy can prepare a drug for an individual patient with a prescription. Under section 503A, the ingredient must be part of an approved drug, have a USP or NF monograph, or be on FDA's bulk drug substances list. [7] Compounded drugs are not FDA approved, and FDA does not verify their safety, effectiveness, or quality before they are dispensed. [8] In 2023 FDA placed a large group of popular peptides in Category 2, meaning it identified significant safety risks and they could not be compounded while under evaluation; many have since moved to a withdrawn list or back into review. [9] Every change is dated on our regulatory tracker.

3. Research chemicals

Websites that sell peptides labeled "research use only" or "not for human consumption" are selling unapproved substances. Federal law requires FDA approval before a new drug is introduced for human use, and the label does not change that when the product is sold to people to inject. [10] There is no assurance of identity, dose, sterility, or purity. We do not link to these sellers.

Evidence is the second filter

Regulatory status tells you how a product is made and sold. Evidence tells you whether it does anything. The two often diverge. BPC-157, one of the most searched peptides, has dozens of rodent studies but no published randomized human trial; a 2019 review concluded human trials are needed before clinical claims can be made. [11] On the other end, semaglutide has trials in thousands of people. [5]

We grade every peptide on a five-step scale: human randomized trials, human observational data, animal only, anecdotal, or none. You can browse peptides by the condition they are studied for and see each one's grade side by side.

A note on size and naming

The 40 amino acid line is a regulatory definition, not a biological law. Some products marketed as "peptides" are proteins by FDA's rule. IGF-1 LR3, for example, is an 83 amino acid engineered analog of IGF-1, well past the threshold. [1] The label on the vial does not decide how a substance is regulated.

Athletes: a separate rulebook

If you are tested, WADA's Prohibited List applies regardless of FDA status. It bans many peptide hormones and growth factors, and it has a catch-all section for substances not approved for human use anywhere, which covers most research peptides. [12] Each peptide page shows its WADA status.

How to use this site

Start with the question you actually have. If it is "does this work," go to the evidence section of the peptide page. If it is "can I get it legally," read the regulatory and access sections. If it is "what should I do," talk to a licensed clinician: we summarize published evidence and public records, and we do not recommend treatments.